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Since 1993, astronomers have used specially instrumented spacecraft to help identify the source of GRBs. These include the Ulysses spacecraft and several spacecraft near the Earth: the BeppoSAX, Wind observatory, the Compton Gamma-Ray Observatory (CGRO) and the Rossi X-Ray Timing Explorer. Unfortunately, these near- Earth spacecraft are too close to each other to allow a definitive triangulation of burst locations. The loss of the Pioneer Venus orbiter and Mars Observer in the early 1990s meant that astronomers lacked a third detector source for accurate triangulation of deep-space GRBs. The addition of the NEAR spacecraft to the interplanetary network greatly increased the probability of associating a GRB with a particular source using optical and radio telescopes. The GRS onboard NEAR was not originally intended to begin its work until the spacecraft reached Eros. However, while en route to Eros, simple software changes to the XGRS system allowed scientists to use the spectrometer for GRB detection. By adding the NEAR spacecraft to the GRB interplanetary network (IPN) and taking advantage of significant improvements in telemetry rate and computational capability, NEAR helped

reduce GRB detection and

triangulation times from months to seconds. As an example, gamma ray detectors on the NEAR and Ulysses spacecraft first recorded gamma ray burst GRB000301C on March 1, 2000.3 5

Initially, the sky coordinates of the burst

were not well-defined, but with data from the NEAR and Ulysses spacecraft, an area of the sky about 4.2 arcminutes wide and 180 degrees in length was identified as the potential source. A second position from the Rossi X-Ray Timing Explorer reduced the error to 4.2 degrees long and 8.7 arcminutes wide. Triangulation of the three data points further narrowed the gamma ray emission zone to within a 5 0 arcminute square, thus allowing a much quicker search of the sky by the HST and ground-based telescopes. Over a 15 -month period from December 1999 to February 2001, the IPN, including NEAR, detected over 100 GRBs.3 6

Of these, 34 were

localized rapidly and precisely enough to allow optical and radio telescope follow-up observa- tions. The suspected GRB emission locations were determined with accuracies of the order of several arcminutes. One of the most interesting results was the detection of a GRB originating in the southern constellation Carina. Optical observations of an extreme red-shift indicated that the source of the GRB was about 12.5 billion light-years from Earth, making it the most distant GRB yet detected.

Unlocking the Secrets of Eros

The NEAR spacecraft entered Eros orbit on February 14, 2000, beginning its one-year mission to explore Eros. Orbital characteristics ranged from elliptical to circular and took NEAR within 35 km [ 22 miles] of the surface of Eros. Then, almost six years to the day after launch, engineers at JHUAPL brought NEAR’s mission to its culmination with a successful controlled descent to the surface of Eros.

Although the primary mission of NEAR was to investigate

the mineralogy, composition, magnetic fields, geology and origin of Eros, NEAR obtained much more detailed information during its orbital encounter with Eros.

Images, laser altimetry and radio-science measurements provided strong evidence that Eros is a consolidated, yet fractured asteroid with a regolith cover varying dramatically in depth from near zero to as much as 100 m [ 328 ft] in some areas.3 7

Scientists believe that

the presence of joined and well-defined craters is indicative of cohesive strength within the asteroid. Surface images show the geometric relationship of grooves and cuts in the surface, suggesting that the rock is competent and not a loosely bound agglomeration of smaller rocks. The gravity field on Eros appeared to be consistent with that expected from a uniform- density object of the same shape. The measured density of Eros indicates that it has a bulk porosity of 21 to 33% , implying that even though the asteroid’s mass is uniformly distributed, it is significantly porous and potentially fractured, but to a lesser extent than Mathilde. Imaging at resolutions of a few centimeters per pixel revealed a complex and active regolith that has been significantly modified and redistributed by gravity-driven slope processes. High-albedo features noted in images taken around crater walls that slope in excess of 25 ° were

often 1.5 times brighter than their

surroundings, indicating recent changes in surface characteristics due to regolith sloughing (above right).3 8 Silicate mineralogy analysis performed by the NIS was consistent with ordinary chondrite meteorites. Spatially resolved measurements of the asteroid’s surface provided no evidence for mineral compositional variation. Scientists believe that the spectral uniformity of Eros may have resulted from a uniformly high degree of space weathering caused by micrometeorite bombardment.

> Close-approach Eros crater w all. Material on the inner w all of the crater in the center of the im age is b righter than the surrounding regolith and is thought to b e sub surface m aterial ex posed w hen overly ing, dark er regolith slid off. The  eld of view is 1. 2 k m [ 0. 7 m iles] across, tak en from 3 8

k m [ 24 m i] ab ove Eros. ( Im ages courtesy of NASA/ J HUAPL. )

The NEAR spectrographs, XRS, GRS and NIS measured the elemental and mineral composi- tion of Eros. Data acquired by the XRS during orbiting showed calcium, aluminum, magnesium, iron and silicon abundances consistent with ordinary chondrite and certain primitive achondrite meteorites. However, the level of sulfur typical of chrondritic meteorites was absent or depleted on Eros. Although the surface of Eros appears to be

elementally homogeneous, the XRS instrument can measure only surface composition, so it is unknown if the sulfur depletion is a surface effect or consistent through the core of the asteroid. If the sulfur depletion is consistent across the bulk of the asteroid, this would imply an association with primitive achondrite meteorites. The orbital GRS measurements had lower signal levels than predicted, so the elemental ratios with the highest precision were measured after landing. GRS data showed the Mg/Si and Si/O ratios and the abundance of K to be consistent with chondritic meteorite values, but found Fe/Si and Fe/O levels to be lower than what would be expected with chrondritic meteorites. Since these measurements were

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Oilfield Review

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